Replace in-field string metadata with resource-level default fields

[emlun]

This addresses #1643:

We request that you do the following:

• Remove the in-field encoding of language metadata using Unicode tag characters
• Remove the in-field encoding of string direction using strongly directional marks
• Add a document-level field for the document language and a document-level field for default direction; such fields would be optional, since you have an installed base

New language and direction attributes on PublicKeyCredentialUserEntity seems like the most appropriate implementation of a "document-level field", since these are the only natural language string parameters we have. The new attributes won't work with currently existing CTAP authenticators, but their processing is specified as "best-effort" (at least that's the intent) so RPs should be aware that they might not work.

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The following tasks have been completed:

• [ ] Modified Web platform tests (link)

Implementation commitment:

• [ ] WebKit (link to issue)
• [ ] Chromium (link to issue)
• [ ] Gecko (link to issue)

Documentation and checks

• N/A ~~Affects privacy~~
• N/A ~~Affects security~~
• N/A ~~Updated explainer~~

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